Superplastic forming (SPF) is a manufacturing process ehich makes use of the characteristic of certain metals when heated and stretched to undergo elongation of several hundred percent without failure due to local necking.
SPF takes place at a temperature in the region of one-half the melting point of the metal. It is a relatively slow process, with typical elongation rates of 100% per hour. Titanium alloys, nickel alloys, and aluminium alloys and some stainless steels possess the necessary characteristics for superplastic forming.
The usual SPF process involves placing a sheet of the superplastic material in a die, heating the material to a temperature at which it exhibits superplasticity, and then using a gas to apply pressure to one side of the sheet. Sufficient pressure is applied to strain the material at a strain rate which is within the superplasticity range of the material being formed at the selected temperature. This gas pressure creates a tensile stress in the plane of the sheet which stretches the sheet and causes it to form into the die cavity.
A disadvantage of superplastic forming is the high cost of manufacturing dies, which are usually made of steel, since they must have accurately formed cover and die parts for perfect sealing to the blank and with optimally located inlet/outlet gas supply bores with the necessary unions for connection to external gas supply/exhaustion pipes. Also, separate dies are required for producing each component and the task of disconnecting one die and installing another into a superplastic forming apparatus is very time consuming. It is known from GB No. 1,495,655 and U.S. Pat. No. 4,584,860 to reduce the cost of manufacturing dies by making them from ceramic material but even so the cost of die production is still high because of the need to provide heating elements and gas supply ducts in the die. Also, the ceramic material cannot generally withstand the high pressures that are exerted in the superplastic forming process.
It is known to perform SPF in a special containment vessel that can contain removable dies and this greatly reduces the cost of making the dies since it is not necessary to provide gas connection ducts in the die itself but rather such ducts are provided in the containment vessel and this makes the manufacture of the dies considerably less expensive. However the use of a removable die is not known in the technique of `back pressure forming` described below and the use of such dies in back pressure forming provides advantages that are not apparent from their use in normal superplastic forming.
It is also known to perform simultaneous superplastic forming and diffusion bonding (SPF/DB) by compressing a mould in a hot platen press, i.e. a press having heatable platens. Heat from the platens heats the SPF/DB mould to the desired temperatures to perform SPF/DB; also, the press holds the mould parts together to withstand the pressure within the mould.
One undesirable characteristic of superplastic materials is their tendency to cavitate, i.e. to form small internal voids, during the tensile deformation imposed by the forming operation. A known method (known as `back pressure forming`) for overcoming this problem involves applying a pressure to both sides of the superplastic material or blank during forming. This reduces the magnitude of tensile stresses acting on the void nucleation sites, thus preventing the formation fo voids or decreasing their size and number. Known SPF apparatus, e.g. from U.S. Pat. No. 4,516,419, comprise two halves, between which a blank of forming material is sealably sandwiched. Each half of the mould has an inlet/outlet orifice through which an inert gas such as argon is passed under pressure. The blank of material is heated to a temperature at which it exhibits superplastic properties and gas pressure is applied simultaneously to both halves of the mould. After a suitable length of time th pressure in the lower half of the mould is reduced in accordance with a predetermined pressure/time variation and the excess pressure in the upper half of the mould forms the metal blank into the shape of the lower half of the mould. Alternatively, the pressure in the upper half of the mould can be increased in accordance with the predetermined prssure/time variation whilst the pressure in the lower half of the mould is held constant. This achieves the same effect. An example of this alternative method is described in U.K. Patent No. 2,100,645.
According to both these patents, the pressures on the respective sides of the superplastic sheet are controlled to provide the stress to form the sheet superplastically while avoiding formation and growth of voids or cavities in the structure of the blank as it is forming. These opposing forces cause relatively uniform thinning of the blank as well as alleviation of cavitation. The differential (or forming) pressure requires continual control and adjustment during the forming cycle and must reflect the physical characteristics of the material of the blank, the die shape and the forming temperature. In order to follow these pressure-time profiles accurately, valves controlling the input or exhaustion of gases to both sides of the metal blank must either be operated manually by a skilled operator or be controlled by a computer or micro-processor.
A further problem with the technique of back pressure forming is that towards the final moments of the forming process pressure fluctuations can occur inside the die which ruin the resulting formed product by causing it to ripple or buckle while it is still in a soft superplastic state. As the metal blank forms, it gets closer to the walls of the die and it may obstruct the inlet/outlet orifices of the die depending on their location. Where the die comprises a simple `bowl` shape the orifice in the die side of the blank will conveniently be located in the very bottom of the die. If a blockage occurs it will cause a fluctuation in pressure inside the die which can damage the moulded article as it is formed. Damage can also occur during completion of a moulding due to the volume of gas remaining in the inlet/outlet pipes which may blow back as these are disconnected from the gas supply, causing localised distortions in the still soft component.
It is an object of this invention to provide a back pressure forming apparatus having provision for connection to a gas supply/exhaustion apparatus such that accurate control of the differential pressure/time profile is readily achievable.
It is a further object of this invention to provide a superplastic forming apparatus and a method which is effective to prevent possible damage to a superplastically formed article in a back pressure forming apparatus.
It is a further object of the present invention to reduce the tooling costs of the dies.